In recent years, in order to improve the fuel consumption of cars and to reduce the weight of car bodies, there has been an increasing need to use high-strength steel sheets for car bodies to allow for a reduction in steel sheet thickness while improving the dent resistance thereof. Generally, when such high-strength characteristics are imparted to a steel sheet, the workability of the steel sheet is deteriorated. Thus, the demand for steels capable of satisfying both high strength and excellent workability is increasing.
Steels capable of satisfying such requirements include multiphase cold-rolled steels and bake-hardenable steels. Multi-phase structure cold-rolled steel can be easily manufactured, and has a tensile strength in the level of 390 MPa or more. Regardless of its high tensile strength as a material for automobiles, multi-phase structure cold-rolled steel has a high elongation. However, it has a low average r-value as a factor indicating the press formability of automobiles, and comprises excessive amounts of expensive alloying elements such as Mn, Cr and the like, which result in high manufacturing costs.
Bake-hardenable cold-rolled steel acts like mild steel in terms of yield strength upon press forming of steel which has a tensile strength of 390 MPa or less. Thus, bake-hardenable cold-rolled steel has excellent ductility, and spontaneously increases in yield strength during paint baking after press forming. This steel is considered ideal in comparison with conventional cold-rolled steel, which is generally deteriorated in formability as the strength of the steel increases.
Bake hardening is a process which employs a kind of strain aging occurring as interstitial elements dissolved in a solid solution state in the steel, such as solute nitrogen or solute carbon, fix dislocations created during deformation. When the steel has large amounts of solute carbon and nitrogen, the amount of bake hardenability of the steel advantageously increases; however, natural aging properties also increase due to the large amounts of solid solution elements, deteriorating formability. Thus, it is very important to optimize the amounts of solid solution elements in the steel.
Generally, a bake-hardenable cold-rolled steel sheet is manufactured by coiling a low-carbon, P-containing, Al-killed steel at a low temperature of 400˜500° C. and then batch-annealing the coiled steel. Herein, a steel having a bake hardenability of about 40-50 MPa is mainly used. It is known that batch annealing in this manufacturing method can improve both the formability and bake hardenability of the steel.
Meanwhile, the P-containing Al-killed steel that should be subjected to continuous annealing is cooled at a relatively high rate, and thus it is easy to secure the bake-hardenability of the steel. However, there is a problem in that the formability of the steel is deteriorated due to a high heating rate and a short annealing process. Thus, the use of the steel sheet manufactured using batch annealing is limited only to the outer panels of automobiles, which do not require workability.
In recent years, surface-treated steel sheets have been mainly used for the production of automotive parts. In the case of galvanized steel sheets obtained by surface-treating a bake-hardenable steel, if the surface integrity of the steel sheet is not sufficiently ensured, scratch-like defects will be highly likely to occur on the steel sheet surface after a plating process. Also, brilliant surface defects will be highly likely to occur after metal sheet processing.
Such defects are generally formed because Al- and P-based composite oxides, which are formed at the surface layer (within a few μm from the surface) of the steel containing excessive amounts of Al and P during a hot-rolling process, form oxides along grain boundaries or sub-grain boundaries.
Accordingly, in order to overcome the problems of bake-hardenable steels while taking advantage of the bake-hardenable steels, various technologies have been developed. Recently, with rapid advance in steel manufacturing techniques, it has become possible to control the amount of solid solution elements in the steel and to manufacture bake-hardenable sheets having excellent formability by using Al-killed steel sheets containing strong carbide/nitride forming elements, thereby satisfying the demand for bake-hardenable cold-rolled steel sheets, which can be used for the outer panels of the automobiles requiring dent resistance.
Japanese Patent Publication No. Sho 61-026757 discloses an ultra-low-carbon cold-rolled steel sheet, which comprises: 0.0005-0.015% of C; 0.05% or less of S+N; and Ti and Nb or a combination thereof. Japanese Patent Publication No. Sho 57-089437 discloses a method for manufacturing a sheet having a bake hardenability of about 40 MPa or more using a Ti-containing steel comprising 0.010% or less of C. Such methods are techniques of imparting bake hardenability to the steel sheet while preventing deterioration in other properties of the steel sheet by appropriately controlling the amount of solid solution elements in the steel through control of the content of Ti and Nb or the cooling rate during annealing. However, for the Ti-added steel or the Ti and Nb-added steel, it is necessary to strictly control the amounts of Ti, N and S during manufacturing of the steel in order to ensure appropriate bake hardenability, and thus the manufacturing cost of the steel is increased.
Meanwhile, various methods of improving the physical properties of a bake-hardenable steel sheet through addition of alloying elements have been reported. For example, Japanese Patent Laid-Open Publication No. Hei 5-93502 discloses a method for enhancing bake hardenability by addition of Sn, and Japanese Patent Laid-Open Publication No. Hei 9-249936 discloses a method for enhancing the ductility of steel by relieving stress concentration on grain boundaries through addition of V and Nb. Also, Japanese Patent Laid-Open Publication No. Hei 8-49038 discloses a method for enhancing the formability of steel through addition of Zr, and Japanese Patent Laid-Open Publication No. Hei 7-278654 discloses a method for enhancing the formability of steel by increasing the strength of the steel while minimizing deterioration of work hardening index (N-value) through addition of Cr.
However, these methods are merely techniques of improving the bake hardenability or formability of steel and do not disclose a problem of deterioration in aging resistance resulting from an improvement of bake hardenability, and a problem of secondary work embrittlement resulting from an increase in the content of P, which is necessarily added due to an increase in the strength of bake hardenable steel. For example, when P is added in an amount of 0.07% to produce a bake hardenable steel having a tensile strength of about 340 MPa, the ductility-brittleness transition temperature (DBTT) of the steel as a reference to determine the secondary work embrittlement is −20° C. at a draw ratio of 1.9. In addition, when P is added in an amount of about 0.09% to produce a high-strength steel having a strength of about 390 MPa, the steel can have a very low DBTT of 0˜10° C. The above-described methods correspond to a steel having a B content of about 5 ppm, and in these methods, it is considered that the improvement in DBTT by B cannot be achieved, because the content of P is excessively large.
If B is added in an excessive amount in order to improve the secondary work embrittlement resistance of steel, it will deteriorate the properties of the steel. For this reason, the amount of B added is limited.
Since the steel must have a DBTT of −20° C. or lower to prevent secondary work embrittlement and have a DBTT of −30° C. or lower to ensure more stable resistance to secondary work embrittlement, there is the necessity of investigating new components other than B in the bake hardenable steel and new manufacturing conditions.
An aspect of the present invention provides a steel which can simultaneously ensure high strength and resistance to secondary work embrittlement while solving the problems occurring in the prior art, and preferably a high-strength bake-hardenable steel, in which the occurrence of surface defects is suppressed and which has excellent bake hardenability and room-temperature aging resistance and a high bake hardening value, as well as a manufacturing method thereof.